Preparation, structural characteristics, and catalytic performance of Cu

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Kinetics, Catalysis, and Reaction Engineering

Preparation, structural characteristics, and catalytic performance of Cu– Co alloy supported on Mn–Al oxide for higher alcohol synthesis via syngas Lu Zhao, Jiani Duan, Qiulan Zhang, Ying Li, and Kegong Fang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03304 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 12, 2018

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Industrial & Engineering Chemistry Research

Preparation, structural characteristics, and catalytic performance of Cu–Co alloy supported on Mn–Al oxide for higher alcohol synthesis via syngas

Lu Zhao,a,* Jiani Duan,a Qiulan Zhang,a Ying Li,a Kegong Fanga

a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese

Academy of Sciences, Taiyuan 030001, Shanxi, P. R. China.

*Corresponding author: Dr. Lu Zhao

E-mail addresses: [email protected]

Tel/Fax: +86-(0)351-4041153

1

ACS Paragon Plus Environment

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ABSTRACT

A series of Mn-Al oxide supported Cu-Co alloy catalysts having various molar ratios of Cu/Co were prepared using the facile sol-gel synthesis, with the aim of investigating the effects of the chemical, physical properties in the catalytic behavior on higher alcohol synthesis (HAS). The as-prepared samples were characterized through utilizing XRD, BET, TEM, XPS, H2-TPD/TPR, CO-TPD, In-situ CO adsorption DRIFTS, CO-TPSR techniques. The study suggests that the proper ratio of Cu/Co could adjust the amount of adsorbed hydrogen and non-dissociatively adsorbed CO at the surface of catalyst on CO hydrogenation and insertion, improve the reducibility, and ameliorate the catalytic behavior for HAS. Especially, the 3Cu–5Co/(Mn–Al) catalyst showed the best catalytic behavior with a CO conversion of 33.4%, total alcohols selectivity of 39.7%, C2+ alcohols selectivity in the total alcohols of 57.3% at GHSV of 5000 h-1 , 260 ºC and 5 MPa.

KEYWORDS: Cu-Co alloy; Sol-gel technique; CO hydrogenation; C2+ alcohols

2


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1 INTRODUCTION

Due to the potential value of higher alcohols as chemical feedstocks, high-quality clean fuels, and fuel additives, higher alcohol synthesis (HAS) via syngas (a mixture of H2 and CO), which converted from renewable biomass, shale gas, and coal, has become more and more attractive 1-3. However, because of the poor catalytic stability and low space-time yield for alcohol products, the existing technologies are still on the experimental stage 4. At present, for HAS, several types of the catalysts have been reported, namely, modified Cu-based (CuZnAl-based) methanol catalysts 5-7

, noble metal-(Rh-)based catalysts

17-19

and Cu-modified Fischer–Tropsch synthesis (Co-, Fe-, Ni-based) catalysts

12-16

, MoS2- and Mo2C-based catalysts

8-11

, .

Modified Cu-based (CuZnAl-based) catalysts generally obtain butanol and methanol with low selectivity to C2+ alcohols. Although MoS2- and Mo2C-based catalysts exhibit

good resistance to sulfur and have a

comparatively higher catalytic performance, the sulfur compound impurities are introduced inevitably in the final products. Noble metal-(Rh-)based catalysts have higher selectivity to C2 alcohol, but the scarcity and valuableness of noble metal have restricted their commercial development. Among these catalyst systems, Cu-modified Fischer–Tropsch synthesis (Co-, Fe-, Ni-based) catalysts are regarded as potential catalysts for HAS, due to low cost, comparatively higher C2+ alcohols selectivity and reaction activity under mild conditions 20,21. HAS over Cu-modified Co-, Fe-, Ni-based catalysts requires both non-dissociatively and dissociatively adsorbed CO at the surface of catalyst. Co, Fe, and Ni elements are used to catalyse carbon chain growth and CO dissociation, and metal Cu to catalyse CO non-dissociation and insertion

1,5

.

3



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Hence, it is important to keep a delicate balance of CO non-dissociative insertion and dissociative activation for producing higher alcohols. The synergistic function between Cu and Fischer-Tropsch synthesis elements plays the crucial role in HAS. Cu modified Fisher-Tropsch catalysts mainly refer to the Cu modified Feand Co-based catalysts. In case of the Cu modified Co-based catalysts, Cu-Co alloy is true active site for HAS Co+/Co pairs

24

and Co2C/Co

25

22,23

, which is widely accepted, although

have also been suggested as active sites for

higher alcohol formation. Extensive studies accounted that interreaction of two elements favors the Cu–Co alloy formation, which shows comparatively high C2+ alcohols selectivity

26,27

. Moreover, a proper surface distribution of the Cu-Co

alloy active sites also significantly affects the catalytic behaviors

28

.In addition, for

the Co-based catalysts, Al2O3 is the most commonly used support in CO hydrogenation

29,30

. However, the formation of an unwanted CoAl2O4 phase is

enhanced, due to the strong interreaction of Al2O3 and Co ions. This spinel structure restricts Co ions intensely, and Co ions cannot be reduced easily even below a temperature of 800 oC, which inhibits the higher alcohols formation

31

.

In order to restrain the CoAl2O4 phase formation, Mo, Mn oxide can be used for modifying Al2O3

31,32

. Furthermore, Mn species were observed to be beneficial

for significant improvement in CO adsorption, which contributed to the growth of carbon chains and formation of alcohols

33

. Hence, Mn-Al oxide is

considered as a promising supporter of metal Co catalysts for applying to catalytic hydrogenation of CO.

4


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In this study, the effects of the chemical and physical properties on the catalytic behavior of Mn-Al oxide supported Cu-Co alloy having various molar ratios of Cu/Co in HAS were studied by utilizing XRD, BET, TEM, XPS, H2-TPD/TPR, CO-TPD, In-situ CO adsorption DRIFTS, CO-TPSR techniques. Through investigating performance/structure relationship, some positive suggestions were also given for the optimizations and designs of catalysts.

2 EXPERIMENTAL SECTION

2.1 Catalyst preparation

The Mn–Al oxide supported Cu–Co alloy samples having various molar ratios of Cu/Co were prepared using a facile sol-gel synthesis technique. In this study, all reagents were analytical grade, and the molar ratio of Mn/Al was retained in 5/3. Typical procedures for catalysts preparation were provided. Citric acid (CA), Mn nitrate, and Al nitrate (CA/(Mn + Al) = 1.2/1) were dissolved directly with distilled water to obtain the aqueous solution. To this saturated solution, the polyethylene glycol 200 (PEG 200) was added dropwise with mild stirring as an esterifying agent. The PEG 200/CA molar ratio was maintained at 0.6/1. After stirring for 1 h, solution A was obtained. In a next step, Cu nitrate and Co nitrate were dissolved at room temperature with distilled water (Cu/Co = 0, 2, 3, 4, 5, 6, and 8), simultaneously, the (Cu + Co)/(Mn + Al) molar ratio was adjusted toward 2/1. CA was introduced in this solution with (Cu + Co)/CA = 1/1.2, for the purpose of providing the molecular level cations mixture. The solution was stirred at 50 oC for 1 h to obtain the clear solution B. 5



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Under vigorous stirring, solution B and solution A were mixed together. It was evaporated to give a gel after mild stirring the aforesaid solution over 2 h at 80 oC. A xerogel was obtained by drying this gel at 120 oC for 8 h. And then, it was heat-treated for 3 h at 450 oC in air to form the catalyst precursors. The catalysts were named as xCu–(8-x)Co/(Mn–Al), in which the Cu/Co molar ratio is x/(8-x)

2.2 Characterization

The prepared samples were characterized through utilizing XRD, BET, TEM, XPS, H2-TPD/TPR, CO-TPD, In-situ CO adsorption DRIFTS, CO-TPSR techniques. The procedure has been illustrated in detail in our previous studies 30,32.

2.3 Catalyst evaluation

The catalyst precursors (40–60 mesh, 1.5 mL) were mixed with 1.5 mL of quartz, and loaded in a fixed-bed reactor. Prior to the each evaluation, the oxide precursors were in-situ reduced for 3 h at 550 oC in hydrogen atmosphere at constant pressure. When the reactor cooled to 260 oC, the syngas (H2/CO = 2:1, GHSV = 5000 h-1, P = 5.0 MPa) was fed into the reactor. After 24 h of the reaction, Data were all taken at stationary state. The liquid products and outlet gas were analyzed by gas chromatography. The details of gas chromatography analysis were illustrated in our previous studies 16,30.

3 RESULTS AND DISCUSSION

6


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3.1 XRD

The XRD patterns of the 8Cu/(Mn–Al), 8Co/(Mn–Al), various Cu–Co/(Mn–Al) precursors are presented in Figure 1(a). The oxidic precursor of the 8Cu/(Mn–Al) catalyst showed nine peaks with 2θ of 32.5o, 35.5o, 38.7o, 48.7o, 58.3o, 61.5o, 63.8o, 66.2o, and 67.9o, which originated from the (1 1 0), (1 1 1), (1 1 1), (2 0 2), (2 0 2), (1 1 3), (0 2 2), (3 1 1), and (1 1 3) crystal surfaces of CuO (JCPDS#80-1268), respectively. Similarly, the peaks of the 8Co/(Mn-Al) precursor located at 2θ of 31.3o, 36.9o, 44.8o, 59.4o, 65.3o were assigned to the (2 2 0), (3 1 1), (4 0 0), (5 1 1), and (4 4 0) reflections of Co3O4 phase (JCPDS#78-1970), respectively. For the Cu–Co/(Mn–Al) precursors, after calcination, Cu and Co species are combined to form a CuCo2O4 phase (JCPDS#37-0878). It is noted that the CuCo2O4 phase is regarded as a necessary role for the formation of Cu–Co alloy phase, where two elements may be homogeneously mixed in an atom-level. When Cu/Co is 3/5, as shown in Figure 1(a), which is relatively closed to the stoichiometric Cu/Co in CuCo2O4, CuCo2O4 phase is the dominated phase. As the Cu/Co molar ratio increases further, CuO and CuCo2O4 are the major phases in the Cu–Co/(Mn–Al) oxidic precursors. Therefore, in the oxidic precursors, Cu and Co mostly exist in CuCo2O4. The others are in state of a small amount of CuO and Co3O4, respectively. In other words, CuO, Co3O4, and CuCo2O4 are highly distributed within support. In order to form alloy phase during reduction, the uniformly high mixing is considered as a precondition. Moreover, the diffraction peaks identified to Mn, Al oxide or other phases were not found. This result is in accordance with the structure of amorphous Mn, Al oxide. Figure 1(b) displays the XRD patterns of the reduced monometallic Cu and Co catalysts, and various Cu–Co/(Mn–Al) catalysts. The diffraction peaks indexed to 7



Page 8 of 44

metallic Cu and Co are observed clearly for the monometallic 8Cu/(Mn–Al) and 8Co/(Mn–Al)

catalysts.

Moreover,

for

the

Cu–Co/(Mn–Al)

catalysts,

the

characteristic peaks located between two pure metal peaks can be observed (inset in Figure 1(b)). As pointed out previously

34,25

, on the basis of the peaks between the

peaks of pure metal, the existence of alloy could be verified. Furthermore, according to Vegard’s law

35

, the crystal structures of intermetallic alloys are dependent upon

the relative concentrations and atomic sizes of the elements. The lattice parameter a of Cu–Co/(Mn–Al) is smaller than a of monometallic Cu (3.625 Å), bigger than a of monometallic Co (3.550 Å), as shown in Figure 1(c). Moreover, the lattice parameter a increases linearly by increasing the Cu/Co ratio. This is due to the bigger size of Cu atom. And the linear change also indicates the Cu–Co alloy formation 22,36. In addition, in Figure 1(b) of 4Cu–4Co/(Mn–Al), there was a small peak locates on the right of diffraction peak of the Cu–Co alloy, which might be indexed to metal Co. Furthermore, the weak and broad peaks illustrate that the Cu–Co particles are of small nanoscale size and uniformly distributed in support. On the basis of the Scherrer equation, in Table 1, the crystallite sizes were calculated to be 5.0, 5.6, 6.4, 6.9, and 7.7 nm for Cu–Co/(Mn–Al) of which Cu/Co = 2/6, 3/5, 4/4, 5/3, and 6/2, respectively. And the average crystallite sizes were 8.8 nm and 8.1 nm for the monometallic 8Cu/(Mn–Al) and 8Co/(Mn–Al) catalysts, respectively. It reveals that the uniformly dispersed metal atoms could be formed through reduction of the corresponding precursors. Moreover, in Figure 2, TEM analysis verifies that the alloy nanocrystallites were uniformly distributed within support. The STEM images of Cu–Co/(Mn–Al) and EDS mapping of Cu and Co suggest that concomitant existence of two elements were evenly dispersed in support. In addition, we compared the surface areas of the different reduced samples. The surface area of the Mn–Al support 8


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was 87 m2 g-1, and that of Cu–Co/(Mn–Al) was in the range of 49–66 m2 g-1 (shown in Table 1). The reason for a decrease of surface area may be that the Cu–Co particles block the meso- and micro- pores after supporting the active phase.

3.2 TEM

Figure 2 (a) and (b) displays the typical TEM and HR-TEM micrographs of reduced 3Cu–5Co/(Mn–Al). According to the TEM micrographs, the highly dispersed Cu–Co particles could be observed. It indicates that Mn–Al oxide might hinder the crystallite agglomeration. The size distribution of particles centered in the 5–8 nm range. A 0.210 nm interlayer spacing is identical to the lattice fringe of the Cu–Co alloy (1 1 1) 34. Moreover, the corresponding TEM micrographs showed the 0.213 and 0.181 nm interlayer spacings, which are identical to the lattice fringes of the metal Cu (1 1 1) and (2 0 0), respectively

37

. Furthermore, as shown in Figure 2(b), the

interplanar spacings were about 0.204 and 0.178 nm, which are in accordance with the interplanar distance of the (1 1 1) and (2 0 0) planes of metal Co

38

. This TEM

analysis is in an agreement with XRD analysis. The STEM micrographs of 3Cu–5Co/(Mn–Al) after reduction and EDS mapping of Cu and Co suggest that concomitant existence of two elements were evenly dispersed within support, as shown in Figure 2(c). In Figure 2 (d), the corresponding precursor also exhibits the similar distribution features, implying that the Cu–Co/(Mn–Al) catalyst maintains the good structural stability during the reduction process. As shown in Figure 2(e) and (f), the HAADF-STEM-EDS line patterns indicate that the changes of Cu and Co contents along the line position over the particles occur continuous and simultaneous. This illustrates the alloy formation in Cu–Co/(Mn–Al) 9



Page 10 of 44

after reduction. All characterization results verify that calcining and reducing the bimetallic Cu–Co precursors can obtain the highly dispersed Cu–Co alloy particles.

3.3 H2-TPR

The H2-TPR thermograms of the 8Cu/(Mn–Al), 8Co/(Mn–Al), different Cu–Co/(Mn–Al) precursors are displayed in Figure 3. The peaks located at 250–350 o

C can be assigned to the H2 consumptions of CuO to Cu and Co2O3 to CoO 39,40. The

peaks at 450–750 oC are assigned to the remaining CoO to Co 41. It is obvious that the H2 consumptions peaks of Co3+ and Co2+ shifted to lower temperatures for Cu–Co/(Mn–Al) catalysts with increasing the Cu content. The reduction of Co oxide was most probably promoted by Cu species and shifted to lower temperature range that was overlapped with the Cu2+ reduction

42

. Thus, for the Cu–Co/(Mn–Al)

catalysts, the H2 consumption of Co ions in the CuCo2O4 phase occurs at a lower temperature, due to the presence of Cu ions in CuCo2O4 lattice. Moreover, by increasing the Co content, the positions of the H2 consumption peaks of Cu2+ were shifted to higher temperature. This is an indicative of the strong mutual effect of Cu and Co ions, which restrains the Cu2+reduction. The mutual effect of two elements also promotes the alloy formation, which is deemed as active site for HAS via syngas 43

. In addition, the small H2 consumption peaks at about 500 oC, which belonged to

the reduction of Mn2O3 to MnO, were found in Figure 3. Furthermore, the H2 consumption peaks corresponding to Al ions were not observed. Therefore, Al and Mn ions were still at oxide state. These results are in accordance with report 44.

3.4 H2-TPD 10


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Figure 4 displays the H2-TPD analysis of reduced 8Cu/(Mn–Al), 8Co/(Mn–Al), different Cu–Co/(Mn–Al). A blank run of H2-TPD over the Mn–Al support was conducted, and there are not any hydrogen signals. Hence, we could eliminate the influence of the Mn–Al support on hydrogen desorption. On the basis of the literature reports for the Cu- 45 and Co- 46 based catalysts, the general desorbed hydrogen peaks at 50–350 oC were assigned to chemisorbed H species at the metallic state. As shown in Table 1, we distinguished and calculated the amount of hydrogen by using Gaussian function. With increasing the Cu content, the amount of hydrogen exhibits a monotonic decrease. This suggests that the ability for hydrogen activation of the Cu–Co/(Mn–Al) catalysts is adjusted by the Cu/Co molar ratio. The 8Co/(Mn–Al) catalyst shows the maximum value of hydrogen amount, suggesting the strongest H–metal bonds. It is in agreement with highest activity for CO hydrogenation (shown in Table 2).

3.5 CO-TPD

Figure 5 displays the CO-TPD analysis of 8Cu/(Mn–Al), 8Co/(Mn–Al), different Cu–Co/(Mn–Al) after reduction. A blank run of CO-TPD over the Mn–Al support was conducted, and there are not clear CO signals. Hence, we could eliminate the influence of the Mn–Al support on CO desorption. As shown in Table 1, we also calculated the amount of CO by using Gaussian function. For 8Cu/(Mn–Al), the low-temperature CO desorption peak appeared at around 100−350 oC, which was identical to non-dissociated CO desorption. These adsorbed CO species, as reaction precursors, are responsible for HAS, especially for CO insertion. However, for a case 11



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of 8Co/(Mn–Al), the desorbed peak of CO was not found. It can be ascribed to the dissociated adsorption of CO over Co species

47,48

. Moreover, with raising the Cu

content, the desorbed peak area of CO went up gradually, suggesting that Cu/Co molar ratio influenced the CO adsorption strength over the catalytic surface. Generally, the analysis indicates that the amount of the adsorbed CO species over Cu–Co/(Mn–Al) can be adjusted by changing the Cu/Co molar ratio.

3.6 In-situ CO adsorption DRIFTS

Figure 6 presents the in-situ CO adsorption DRIFTS analysis over reduced 3Cu–5Co/(Mn–Al). All the DRIFTS showed two main bands at 2300–2400 cm-1 and 2050–2250 cm-1, which were identical to the vibration of CO2 and adsorbed CO respectively around

49,50

2172

. For adsorbed CO on 3Cu–5Co/(Mn–Al), the absorption bands at and

2116

cm-1

were

assigned

to

C–O

bending

and

stretching ,respectively. Moreover, we found that C–O bond is relatively weak, because of comparatively lower wavenumber for adsorbed CO (the 2200 cm-1 bending and 2140 cm-1 stretching for gaseous CO 51). Furthermore, the CO2 vibration arises at 150 oC, which indicates that C* and O* was formed by dissociating CO, and turned into CO2 at the catalytic surface. As pointed out in the literature 47, metal Co favors the CO dissociation. The formed C* species react with adsorbed hydrogen, converting to CH2 species. And CO could be oxidized to CO2 by O*. In addition, with increasing the reaction temperature, the CO2 amount was rapidly increased. It suggests that the CO dissociation is strongly dependent upon the reaction temperature.

3.7 XPS 12


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XPS spectroscopy of the various elements on the catalyst surface for reduced 8Cu/(Mn–Al), 8Co/(Mn–Al), various Cu–Co/(Mn–Al) was measured. For Cu element, Figure 7(a) shows the Cu 2p3/2 binding energy (BE) was 932.4–932.7 eV, suggesting that Cu species were in the form of Cu0 52. For the Co 2p3/2 BE of Co species, the peaks located at 777.6–777.9 eV could be assigned to the Co 2p3/2 BE of Co0 52, as shown in Figure 7(b). The results are in good agreement with XRD, TEM, H2-TPR characteristics, which exhibit these two species were at the metal state. Furthermore, the Cu 2p3/2 BE of pure Cu of 932.4 eV is lower than that of Cu–Co/(Mn–Al), and the Co 2p3/2 BE of pure Co of 777.9 eV is higher than that of Cu–Co/(Mn–Al), which suggests that the Cu–Co alloy was generated by reducing the precursors related to a difference in electron work function of two species

53

23

. This is

. The electron

transfer from Cu to Co can occur because of comparatively lower electron work function of Cu. Therefore, the BE shifts in the Cu–Co alloy can be found. Moreover, an influence upon the chemical environment of Cu–Co alloy dispersed on Mn–Al support could be predicted, due to the interreaction of the support and Cu–Co alloy. Figure 7(c) and (d) displays XPS spectroscopy of Mn 2p, Al 2p of reduced 8Cu/(Mn–Al), 8Co/(Mn–Al), various Cu–Co/(Mn–Al). These spectroscopies exhibit the similar characteristics. It implies that the chemical states of Mn, Al elements were not changed majorly on the catalyst support.

3.8 Evaluation of catalytic performance in HAS

The

catalytic

performances

of

8Cu/(Mn–Al),

8Co/(Mn–Al),

different

Cu–Co/(Mn–Al) were evaluated for HAS and the results are displayed in Table 2. In 13



Page 14 of 44

addition, a blank run of HAS over the Mn–Al support was also conducted. There is not any CO conversion. Hence, we could eliminate the influence of the Mn–Al support on CO hydrogenation, as shown in Table 2. Comparing the CO conversion of the different catalysts, 8Cu/(Mn–Al) (17.2%) is lower than Cu–Co/(Mn–Al) (23.4–35.0%), and also significantly lower than 8Co/(Mn–Al) (58.5%). Furthermore, CO conversions of various Cu–Co/(Mn–Al) were increased with increasing the Co content. As shown in Table 1 and Figure 4, according to H2-TPD analysis, the amount of adsorbed hydrogen over the Cu–Co/(Mn–Al) catalysts was increased by decreasing the Cu/Co molar ratio. These results indicate that the CO conversion is positively relevant with amount of activated hydrogen species. The more hydrogen species adsorbed, the higher CO conversion. So, the 8Cu/(Mn–Al) catalyst showed the lowest CO conversion with less adsorbed hydrogen, while the 8Co/(Mn–Al) catalyst showed much higher CO conversion with the most amount of adsorbed hydrogen. The selectivity to total alcohols went up with increasing Cu/Co molar ratio over the catalysts. Research on HAS showed that non-dissociatively actived CO is necessary for alcohol formation 54. However, monometallic Co only promotes the hydrocarbons formation due to the CO dissociative activation

47,48

. Therefore, as shown in Table 1

and Figure 5, the ability for CO non-dissociative activation over 8Co/(Mn–Al) is inadequate, causing an obvious decrease in total alcohols selectivity. Hence, high total alcohols selectivity could not be observed on 8Co/(Mn–Al). According to CO-TPD results, with raising Cu content, the amount of non-dissociated CO increased accordingly. Hence, 8Cu/(Mn–Al) is propitious to the non-dissociative activation of CO. It illustrates that a higher ability in non-dissociated CO activation is essential to enhance the total alcohols selectivity. In addition, the CO2 selectivity increased accordingly by increasing the molar ratio of Cu/Co. This agrees well with the finding 14


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that metal Cu favors the water-gas shift (WGS) reaction 16. The C2+ alcohols selectivity increased from 34.2% with Cu/Co = 6/2 to 57.3% with Cu/Co = 3/5, and then declined to 55.2% with Cu/Co = 2/6. The tendency can be attributed to reasons given below. On the basis of the mechanism of CO insertion, which is gaining widespread acceptance for CO hydrogenation to alcohols, HAS on Cu-Co catalysts needs both dissociatively and non-dissociatively adsorbed CO species at the catalytic surface. Metal Co is used to catalyse carbon chain growth and CO dissociation, while metal Cu to catalyse CO non-dissociation and insertion. Therefore, a mutual effect of Co and Cu species plays a necessary role in HAS, and a proper balance between non-dissociated CO insertion and dissociated CO activation should be kept. The 3Cu–5Co/(Mn–Al) catalyst showed the highest C2+ alcohols selectivity. This can be attributed to the fact that most Cu and Co are present at alloy active state, while Cu and Co species are highly distributed. It could enhance a mutual effect between Cu and Co species significantly, which contributed to this improvement of C2+ alcohols selectivity. Furthermore, although a small amount of alone Cu and Co species were formed after reduction, the high selectivity to C2+ alcohols can be kept at a relatively high level. It may be ascribed to the close contact between two species. The synergetic catalytic effect between the close-contacted Cu and Co species can be maintained during the CO hydrogenation reaction, although phase separation could happen. The highest selectivities for C2+ alcohols (57.3%) and total alcohols (39.7%) are displayed over 3Cu–5Co/(Mn–Al), with STY (space-time yield) of C2+OH of 63.0 mg mL-1 h-1, the selectivity to CO2 of 14.1% and selectivity to hydrocarbons of 46.2%. In addition, although 8Co/(Mn–Al) has higher selectivity of C2+ alcohols due to its high ability in carbon chain growth, total alcohols selectivity was very low (

Preparation, structural characteristics, and catalytic performance of Cu

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